The present invention relates to satellite communication systems, and a method of operating such systems. The invention particularly relates to satellite communication systems and related methods where an earth based user terminal communicates with a satellite which in turn communicates with an earth station, and vice versa. Most particularly, the present invention relates to such a system and its related methods where there is a requirement for the location of the user terminal, on the surface of the earth, to be known to the earth station.
It is known, in a satellite communications system, for a user terminal, perhaps in the form of a radio telephone handset, not dissimilar to a cellular radio telephone handset, to communicate with an earth station, via a satellite, to establish a telephone call or to receive a telephone call, by means of the earth station linking into the terrestrial wire and cable system or into another radio telephone system.
There are twin purposes relating to establishing the exact position of the user terminal on the surface of the earth. Firstly, in order to know how to direct a radio signal to or from a user terminal, when required, from particular satellite at a particular time, it is necessary to know the approximate location of the user terminal so that the appropriate beam from the appropriate satellite can be selected to cover the portion of the Earth""s surface where the user terminal is located. Secondly, in a satellite communication system, in order that call barring, local billing or other restrictions based on the territory wherein the user terminal may be operated can be observed, it is necessary to determine the location of the user terminal with sufficient accuracy for the necessary restrictions to be imposed.
It is known to provide a user terminal where the individual terminal employs xe2x80x9cGlobal Positioning by Satellitexe2x80x9d (GPS) to determine, with some great accuracy, the position of the user terminal on the surface of the earth. The user terminal then transmits, to the earth station, via the satellite or satellites involved in communications, its exact position which is then used by the earth station, in subsequent interactions with the user terminal, to control the fiscal and mechanical aspects of the communication activity with the user terminal. An example of such a system is to be found in European Patent EP 0562 374 by Motorola Corporation filed 27th Mar. 1993.
Such systems require multiple frequency capability from the handset, together with enhanced complication of the handset, in order that the handset may be capable both of communications and of GPS measurements.
It is advantageous, therefore, to provide a system and method whereby the position, on the surface of the earth, of the user terminal or handset can be determined with sufficient accuracy for communication and fiscal purposes without undue complication of the handset or user terminal and without the necessity of the provision of or access to a separate satellite system concerned with GPS.
The present invention seeks to provide a solution to the problem of determining the location of a user terminal or handset, on the surface of the earth, when interacting with a satellite communication system, without the necessity to resort to a solution involving GPS and with sufficient accuracy for the operational and fiscal requirements of a satellite communication system.
In another system, it is merely necessary for the satellite to receive a transmission from the user terminal, in response to the earth station, via the satellite, requesting radio communication from the user terminal, for the earth station to use the delay in response from the user terminal for ranging purposes and to use the doppler shift on the received frequency from the user terminal, together with a foreknowledge of the position and velocity of the satellite, to determine, with a certain degree of precision, the position of the user terminal on the surface of the earth.
Unfortunately, if the cost of the user terminal is to be kept at realistic limits, consistent with the trade in handheld mobile radio telephones, the accuracy of the crystal clock or other frequency source within the user terminal cannot be made consistent with sufficient accuracy of determination of the position of the user terminal, on the surface of the earth, for the functional and fiscal aspects connected with operation of a satellite telephone communication system.
The present invention seeks to provide a method and system whereby the position of a user terminal, on the surface of the earth, can rapidly be determined without multiple frequency capability in the user terminal and with a high degree of accuracy.
According to one aspect, the present invention consists in a satellite communications system wherein a user terminal is operative to transmit to one or more satellites and wherein each of said one or more satellites is operative to transmit to said user terminal, each of said one or more satellites being operative to send and receive signals from an earth station: said system being characterised by said earth station being operative to exchange signals with said user terminal through said satellites and thereafter, being operative to analyse said signals to determine the position of said user terminal on the surface of the earth.
According to a second aspect, the present invention consists in a method for determining the position of a user terminal on the surface of the earth by employing one or more satellites each operative to send and receive messages from an earth station and each operative to send and receive messages, from said earth station, to said user terminal, said method being characterised by the steps of: said earth station exchanging messages, via said one or more satellites, with said user terminal and, therafter, said earth station analysing said returned messages to determine the position of the user terminal.
The invention further provides a method and system wherein, when said one or more satellites comprises just one satellite, said exchanged messages between said user terminal and said ground station are adapted to measure the doppler shift due to motion of said one satellite relative to said user terminal and to measure the radio propagation delay between said ground station and said user terminal.
The invention further provides a method and system wherein said just one satellite is operative to communicate with said user terminal using one out of a plurality of beams, each of said plurality of beams being interactive with a respective one out of a plurality of areas on the surface of the earth, ambiguity of position of said user terminal being resolved by observation of with which out of said plurality of beams said user terminal exchanges said signals.
The invention yet further provides a method and system wherein, when said one or more satellites comprises more than one satellite, said exchanged signals between said earth station and said user terminal are adapted to measure the propagation delay between said earth station and said user terminal via each of said more than one satellites.
The invention still further provides a method and system wherein each of said more than one satellites is operative to provide a broadcast message at a predetermined time, and wherein said user terminal is operative to measure and record the time of arrival of each of the broadcast messages and to report back to said earth station said time of arrival of said each of said broadcast messages.
The invention still further provides a method and system wherein said earth station is operative to send out a message, via each of said more than one satellites, and where said user terminal is operative to return a message within a predetermined time of receipt of said message via each of said more than one satellite, said earth station being operative thereby to calculate the propagation delay between said earth station and said user terminal via each of said more than one satellites.
The present invention still further provides a method and system wherein said earth station is operative to send said message via said each of said more than one satellites an optimum number of times, dependently upon the estimated position of said user terminal with respect to said each of said more than one satellites, and to take the average of the propagation delays derived therefrom.
The present invention yet further provides a method and system wherein said user terminal is operative to detect and record the time of arrival of broadcast messages from satellites which are no longer in sight and to report said previous broadcast messages to said earth station, said earth station using knowledge of the position of said satellites, no longer in sight, at the time of receipt of the broadcast message by said user terminal to assist in the calculation of the position of said user terminal.
The present invention still further provides a method and system wherein said user terminal is operative to note the apparent recorded time, reported by said user terminal, between two known intervals and is operative thereby to correct for drift and offset error in the timer in said user terminal.
The present invention further provides a method and system wherein said earth station is operative to send a signal at a known frequency to said satellite and wherein said satellite is operative to use an internal oscillator to transpose said signal of a known frequency and return the transposed signal to said earth station on a plurality of other frequencies, said earth station being operative to measure the other frequencies and to derive therefrom the doppler shift between said earth station and said satellite and to derive the error in the internal oscillator in said satellite.
The present invention further provides a method and system wherein said user terminal is operative to eliminate error in its own internal oscillator by employing said internal oscillator to convert the frequency of a signal, received from said satellite, in a first direction, and thereafter to convert the frequency of said signal in a second direction, opposite to said first direction, thereby eliminating error in said internal oscillator.
The present invention still further provides a method and system wherein said earth station is operative to exchange said messages with said user terminal, via said just one satellite, a first optimum number of times to establish, by averaging, said doppler shift and a second optimum number of times to establish, by averaging, said propagation delay, said in orthogonal orbital plains. First and second optimum number of times being dependent upon the estimated position of said user terminal with respect to said just one satellite.